Charge-transfer absorption, emission, and the open-circuit voltage of organic solar cells (Presentation Recording)

Abstract

Electronic processes at the organic hetero-interface limit the photocurrent and photovoltage of organic solar cells. While devices with incident-photon-to-extracted-charge conversion yields of over 85%, and absorbed photon-to-extracted-charge conversion yields of 90-100% have been achieved, the difference between the optical gap of main absorber and open-circuit voltage (Voc) is much larger than for inorganic solar cells. The main improvements in the Voc of organic solar cells have so far been made by tailoring the donor-acceptor interfacial energetics, taking advantage of well-known principles of molecular design. Nevertheless, for most material systems we consistently find a large (>0.55 eV) difference between eVoc and the energy of the intermolecular charge transfer (CT) state. We present experimental evidence that this difference can be reduced by reducing the physical interfacial area available for free carrier recombination. We quantify this by analyzing the strength of the interfacial CT state absorption and emission signal at photon energies below the optical gap of the neat materials. We further discuss the influence of the measured electronic coupling, molecular reorganization and non-radiative recombination pathways on Voc. This work opens up unexplored possibilities for increasing the Voc of organic solar cells, bringing it closer to the optical gap of the main absorber.